The present invention relates to a method for encoding analog signals for storage or transmission purposes in which sampled momentary values of an analog signal are converted into pulse-code modulation (PCM) code words in an encoder and PCM difference code words are formed from two successive PCM code words by means of subtraction, and in particular to such a method in which each of the PCM difference code words representing a pulse group, together with at least one additional auxiliary code element, form a code word having a selected constant length, which is designated as a reference code word.
A particularly high accuracy and reliability for the storage or transmission of encoded analog signals can be achieved by the use of uniformly quantized pulse code modulation. In order to reduce the number of elements or bits in the code words, it is known to utilize so-called difference pulse code modulation in which only the amplitude changes of successive momentary values of the analog signal are stored or transmitted in digitally encoded form. Within specified quality parameters, the number of code elements per difference PCM code word is determined by the maximum possible change of the analog amplitude between two successive momentary values of the analog signal. Theoretically, it is possible to reduce the transmission or memory capacity of the system by selecting the number of bits in a code word to be only as large as is required for the representation of the size of the sampled momentary analog value. This, however, requires additional code elements in each code word. During the storage or transmission of code words having differing bit lengths, these additional code elements may be lost due to bit errors, thereby causing distortion and inaccuracy in the recovery of the original analog signal.
As an attempted solution to this problem, it is known to employ PCM redundancy reduction methods which utilize a constant code element amount for the transmission of the momentary signal values, the constant code element amount being reduced in comparison to a maximum value which arises as a result of uniform quantization. The binary encoded momentary values, either individually or in blocks, are transmitted in companded form and are identified by means of corresponding auxiliary code elements. In this solution, however, the quality of the recovered original analog signal is dependent upon the degree of companding, so that the attempted reduction of bit errors occurs at the expense of the introduction of an additional error source.
The theory and practice of the use of so-called difference pulse code modulation is described, for example, in the article "Digitization of Audio: A Comprehensive Examination of Theory, Implementation, and Current Practice", by Barry A. Blesser, Journal of the Audio Engineering Society, October 1978, Volume 26, Number 10, pages 739-771; and in the article "Digital Sound Signals: Tests to Compare the Performance of Five Companding Systems for High-Quality Sound Signals", by N. H. C. Gilchrist, BBC Research Department Report 1978/26, August 1978.